Fiber reinforced composite flywheels have been proposed for use as energy storage systems for vehicles and as auxiliary power sources for utilities. However, a practical reasonably priced fiber composite flywheel has been difficult to achieve.
Fiber reinforced composite flywheels have been manufactured by filament winding, and by the hand lay up of multiple preimpregnated plys, for example, as described in U.S. Pat. Nos. 4,102,221 and 4,266,442.
Such composite flywheels exhibit high stiffness and strength in the hoop direction of the flywheel, and low weight relative to metal flywheels. However, such fibers are highly anisotropic, as the transverse stiffness and strength is typically much lower than the longitudinal stiffness and strength. Therefore, fiber reinforced composite flywheels are typically constructed of fiber wound predominately in the circumferential or hoop direction, which reduces radial growth at high speeds.
The growth characteristic is of particular importance as a flywheel rim of one material is commonly bonded to a hub of another material. As speed increases, the differential growth between the rim and hub may cause separation between the two.
Another important characteristic is radial stress which limits the flywheel rotational speed and thus the flywheels' energy storage capacity. The weaker transverse (radial) strength of the composite makes this a key parameter in flywheel design, as the fibers may separate circumferentially along the axis of the fiber.
To overcome these problems, it has been attempted to preload the fibers, to have an initial state of radial compression, as discussed for example in U.S. Pat. No. 5,285,699. However, there are limits to the amount of preload that can be achieved.
In U.S. Pat. No. 5,452,625, annular shaped inserts are interleaved and bonded with annular fiber reinforced composite disks, the annular inserts having a high radial strength substantially greater than the composite section radial strength, to increase the overall radial strength of the assembly. But the difference in stiffness properties, thermal expansion, and growth characteristics at high speed provides a potential for delamination between these disks.
A spiral woven fiber reinforced flywheel rim has been considered,however, the prior efforts have not gained acceptance.